Reusable injectors which can be reloaded with a new medicament container when a prior medicament container is emptied are known from prior art. For example, pen injectors for insulin are commonly known.
WO-2004/002556 discloses a re-usable pen injector for use with containers. The device comprises a dose setting mechanism for setting a required dose to be injected and a drive mechanism having a plunger rod acting on a plunger in the container. When the container has been emptied to a pre-defined extent, the plunger rod can be reset to an initial position by unscrewing the two housing parts. If a user of such prior art injectors sets a dose that is larger than the remaining amount of medicament in the container this will only become apparent to the user when the container is completely emptied, i.e. the plunger rod comes to a stop. This brings about at least two problems, the first being that the user has to reload the device with a new container and immediately perform another injection which can be perceived as inconvenient and tiresome. The second being the fact that even if the user is prepared to perform such subsequent injection, the user often does not know how much medicament was injected in the first injection and thus does not know how much has to be injected in the second injection which can endanger patient safety.
U.S. Pat. No. 5,925,021 describes an injection device which has a microprocessor coupled to it. This microprocessor has among other things the function that it prevents a user from setting a larger dose than the amount of medication left in the container. A major drawback is that it relies on batteries to function, which also impairs reliability of service.